Isolated mesenteric arteries from pregnant rats show enhanced flow-mediated relaxation but normal myogenic tone.

1. The mechanisms of vascular adaptation to pregnancy remain to be fully elucidated. In this study we have compared flow-induced dilatation and myogenic responses in fifth generation mesenteric arteries from late pregnant and non-pregnant Wistar rats. 2. Myogenic tone, assessed as the difference between internal diameter (i.d.) in the presence and absence of external calcium, at each of six transmural pressure steps (40-90 mmHg) was no different in arteries from pregnant and non-pregnant animals (90 mmHg; pregnant animals, 20.5 +/- 3.2% change in i.d., n = 13; non-pregnant animals, 14.4 +/- 4.4% change in i.d., n = 13; n.s.). 3. Flow-induced dilatation was significantly increased in arteries from pregnant animals when compared with that in non-pregnant animals (pregnant, 11.2 +/- 3.7% increase in i.d., n = 12; non-pregnant, 1.2 +/- 1.0%, n = 8; P < 0.02; at the maximal flow rate of 1.46 microliters s-1). 4. Flow-induced dilatation was significantly reduced in arteries from pregnant rats after incubation with the nitric oxide synthase inhibitor N omega-nitro-L-arginine methyl ester (11.2 +/- 3.7% increase in i.d. vs. 0.8 +/- 1.5% with L-NAME, n = 12, at a flow rate of 1.46 microliters s-1; P < 0.02), but not altered in non-pregnant rats (1.2 +/- 1.0% vs. 1.4 +/- 1.5% with L-NAME, n = 8; n.s.). This was also reflected in a greater response to shear stress in the arteries from pregnant animals. 5. We conclude that myogenic tone is not significantly different in pregnant and non-pregnant resistance sized mesenteric arteries of Wistar rats. However, flow-induced dilatation, mediated by nitric oxide release, may play a major role in lowering vascular resistance during pregnancy.